The rate of granulocyte production in man is frequently altered during immunologic reactions to disease. For this reason it has long been thought likely that specific cells within the immune system may contribute to the regulation of granulopoiesis. This notion has been strengthened by observations that both monocytes and T-lymphocytes secrete glycoprotein growth factors, known as granulocyte-monocyte colony stimulating factors (GM-CSF's) which promote both the proliferation and the differentiation of granulocyte progenitor cells in vitro and which may operate in vivo to modulate granulopoiesis physiologically. We intend to study the kinetics of the production of GM-CSF's by both monocytes and T-lymphocytes in detail, using cells isolated from the blood of healthy volunteers. By using endotoxin to stimulate GM-CSF synthesis and secretion by monocytes, we plan to study the initial physiologic events involved in activation of the cell by this complex lipid probe, including the kinetics of the binding of endotoxin to the monocyte surface and potential alterations in the physical properties of the membrane induced by the presence of endotoxin. Using lectin-stimulated mononuclear cell suspensions and mixed lymphocyte cultures as in vitro models of the immune response, we shall attempt to investigate monocyte-T lymphocyte cooperativity in the production of GM-CSF's in response to immunologic activation, and we propose to examine the functional subclasses of T-lymphocytes involved in this response by means of fluorescence-activated cell sorting. We hope that these studies will provide further insights into the physiologic mechanisms involved in the humoral regulation of the proliferation and differentiation of granulocytes.